Adaptation is the adjustment of an organism to its environment. The ability of an organism to adapt to sensory input, such as a smell, is well documented. Research has indicated that cross-adaptation may also occur. For example, an organism may cross-adapt to the sensory input of an odor. In such cross-adaptation, the perceived intensity of one odor may be decreased by subjecting or exposing the subject to a different odorant. Such cross adaptation may occur with perceptually similar odorants (perceptual analogs) and/or with structurally similar odorants (structural analogs).
Psychophysics can address structure-activity relationships in olfaction through the study of cross-adaptation; the decrease in sensitivity to one odor after exposure to a different odor. Cross-adaptation has been interpreted as a measure of the degree to which odors share common sensory channels (Engen, The Perception of Odors, Academic Press, New York (1982); Moncrieff, "Olfactory adaptation and odour likeness," J. Physiol., 133:301-316 (1956)).
In comparing different odorants, a useful distinction can be made between perceptual and structural similarity. Perceptual analogs, i.e., compounds with similar smells, but not necessarily similar chemical structures, have been the focus of most research in the area of olfactory cross-adaptation. The evidence is clear that, despite great dissimilarities in chemical structure, perceptually similar odorants can produce cross-adaptation (Cain, "Odor intensity after self-adaptation and cross-adaptation," Percept. Psychophys., 7:271-275 (1970); Engen & Lindstrom, "Cross-adaptation to the aliphatic alcohols," Amer. J. Psych., 76:96-102 (1963); Moncrieff, supra (1956); Pierce, Wysocki & Aronov, "Mutual cross-adaptation of the volatile steroid androsterone and a non-steroid perceptual analog," Chemical Senses 18:245-56 (1993); Todrank, et al., "The effects of adaptation on the perception of similar and dissimilar odors," Chem. Senses, 16:467-482 (1991)). Further, cross-adaptation is more likely to occur with compounds that share all or most of their perceptual characteristics rather than a single trait (Moncrieff, supra (1956). Work with perceptual analogs suggests that structurally different odorants may stimulate the same sensory channels.
There is less available evidence concerning cross-adaptation among structural analogs; compounds with similar chemical structures, but having different odors. Cross-adaptation has been obtained in perceptually dissimilar compounds (Koster, "Adaptation and cross-adaptation in olfaction," Doctoral dissertation, University of Utrecht (1971)). For example, one study has reported significant cross-adaptation among a homologous series of aliphatic alcohols differing in carbon-chain length (Engen, supra (1963). Although significant cross-adaptation was observed, the degree of cross-adaptation did not vary in relation to the degree of physical similarity of the alcohols. Interpretation of the results was somewhat complicated in that several of the compounds in this study shared some perceptual similarities.
Another study reported cross-adaptation between the structurally similar, but perceptually distinct, n-propanol and n-pentanol (Cain, supra (1970)). Although these odorants were closely matched for intensity, the degree of cross-adaptation was asymmetric; pentanol more completely cross-adapted propanol than vice versa.
Although these studies suggest that structural similarity may be sufficient to yield significant cross-adaptation in perceptually different odorants in specific circumstances, the precise relationship remains obscure. The extent to which chemical structure similarity, in the absence of perceptual similarity, influences cross-adaptation is unknown.
Recently it was found that the (E) and (Z) isomers of 3-methyl-2-hexenoic acid (3M2H) are present in human secretions, especially male underarm sweat. More particularly, the (E)-isomer (E3M2H) has been identified as a major component in male underarm sweat (Zeng, X-N., et al., "An investigation of human apocrine gland secretion for axillary odor precursors," J. Chem. Ecol. 18:1039-1055 (1992); Zeng, et al., "Analysis of characteristic odors from human male axillae," J. Chem. Ecol. 17:1469-1492 (1991)). The (Z)-isomer (Z3M2H) is present at approximately 1/10 the concentration of the (E)-isomer.